A. Field of Invention
The invention relates to the field of performance enhancing shoe components and methods for making the same. More specifically, this invention relates to the field of performance enhancing shoe components which provide energy return, and methods of making the same.
B. Description of the Related Art
The field of athletic shoe manufacturing and sales has become highly competitive in recent years. Various features have been included in shoes for promotional reasons or for comfort. Most improvements in athletic shoes have been made in the physical and structural make-up of the interior and exterior elements of soles of athletic shoes to improve the comfort and appearance of the shoes. While it has long been desired to provide athletic shoes which provide meaningful increases in athletic performance, few, if any, proposals for changes in athletic shoes have produced measurable improvements in performance. Often an improvement in athletic shoe performance is sought but is not attained because the design of the resulting shoe becomes a compromise between comfort and cost.
There is a high demand for athletic equipment which enhances the performance of athletes. As athletic events become more competitive, small improvements in performance become increasingly important. Athletic equipment which enhances the performance of athletes by only a few tenth or hundredths of a second, or a fraction of a centimeter, may provide the "winning edge" sought by many athletes. For this reason, athletes are constantly seeking equipment which will improve their performance. The designs of all types of equipment from bicycles to tennis rackets are constantly scrutinized for features which may improve performance. For example, racing bicycles have been radically redesigned to improve performance while reducing weight and friction. Cycling helmets have also been redesigned to reduce weight and air friction.
This high demand for performance enhancing athletic equipment includes the art of athletic shoes and shoe components. Prior to the present invention, few shoe components have been made available which confer a significant enhancement in athletic performance by providing energy return. No prior shoe feature provides substantial energy return to the user. Similarly, no prior shoe component feature provides energy return in the form of lift, spring or bounce.
The material most frequently used for insoles in athletic shoes is ethylene vinyl acetate ("EVA"). EVA is a lightweight, inexpensive, sponge-like material which does not provide significant energy return. Examples of shoe insoles made from EVA are disclosed in U.S. Pat. No. 4,418,483, which is incorporated herein by reference. This patent teaches making shoe midsoles by combining EVA with a styrene-butadiene rubber. While such midsoles have excellent cushioning properties, they do not enhance the athletic performance by providing energy return to the user. Rather, the cushioning effect of the EVA material can reduce athletic performance by absorbing energy.
U.S. Pat. No. 4,418,483 also discloses combining 1,4-polybutadiene with other rubbers, such a 1,2-polybutadiene, to make hard shoe outsoles or shoe midsoles. These other rubbers do not provide substantial energy return, and reduce the energy return characteristics of the 1,4-polybutadiene. For example, in its cured state, 1,2-polybutadiene is a hard material which lacks the ability to be compressed and rebound to provide energy return. In its uncured state, 1,2-polybutadiene (syndiotactic) may exhibit mild elastomeric properties. When high energy return rubbers such as 1,4-polybutadiene rubber are combined with 1,2-polybutadiene, the 1,2-polybutadiene typically controls the characteristics of the resulting composition and reduces the energy return of the 1,4-polybutadiene.
U.S. Pat. No. 5,147,589, which is incorporated herein by reference, discloses making shoe soles of a polymer blend which is a mixture of rubbers including thermoplastic elastomers, styrene-butadiene rubber, acrylonitrile-butadiene rubber, and plastics such as polystyrene, EVA, or polyvinyl chloride. When high energy return rubbers such as 1,4-polybutadiene rubber are incorporated into such shoe soles, the high energy rubbers also comprise a small weight percentage of the total rubber in the shoe, and, therefore, the shoes do not provide substantial energy return.
While the energy return of 1,4-polybutadiene, in the form of super balls, for example, is known, those of ordinary skill in the shoe art have not looked to 1,4-polybutadiene as a material for shoe components by which athletic performance could be substantially enhanced. In particular, the makers of athletic shoes have failed to appreciate the significant energy return available from shoe components comprising 1,4-polybutadiene in combination with natural rubber, synthetic isoprene rubber, polyisoprene, butadiene acrylonitrile rubber or ethylenepropylene diene modified rubber. This is due in part to the fact that 1,4-polybutadiene in its cured skate has a cheesy texture, and crumbles and tears readily due to a lack of tensile strength. The present invention provides ways by which 1,4-polybutadiene can be cross-linked with other rubbers to manufacture cross-linked compositions which significantly enhance athletic performance when placed within a shoe and which possess both durability and tensile strength, a result which would not be expected given the teachings of the prior art.